7. Looping and counting

The following program counts the number of times the letter a appears in a string: 

word = 'banana'
count = 0
for letter in word:
    if letter == 'a':
        count = count + 1
print(count)
This program demonstrates another pattern of computation called a counter. The variable count is initialized to 0 and then incremented each time an a is found. When the loop exits, countcontains the result—the total number of a’s.

As an exercise, encapsulate this code in a function named count, and generalize it so that it accepts the string and the letter as arguments.

Then rewrite the function so that instead of traversing the string, it uses the three-parameter version of find from the previous section.